1. Field of the Invention
The present invention relates, in general, to liquid storage tanks and more specifically to means for preventing overfill spills in underground liquid storage tanks.
2. Description of the Prior Art
A common problem in filling liquid storage tanks, particularly in the filling of underground tanks in petroleum fueling facilities when product is discharged from aboveground tank trucks, is the spilling of some product, usually relatively small amounts, e.g., 5-10 gallons.
Such spilling can occur if the underground tank is completely filled and there is still product left in the tank truck. Product will rise in the vent pipe of the tank (and the vapor recovery line--if one is used), and the tank truck fill hose will also be full of product.
When the tank truck fill hose is disconnected from the underground tank fill pipe, unless "dry break" fittings are used on the hose and on the fill pipe, enen though the truck valve has been closed, product in the hose will spill out into the fill pipe manhole opening, and product in the vent pipe and/or vapor recovery lines will "seek its level" and drop down to an elevation equal to that of the top of the underground tank fill pipe, spilling product out over the top of the fill pipe into the fill pipe manhole opening.
Prior methods of preventing such spills include allowing only a limited, pre-determined meterd flow of liquid into the tank (the exact amount needed is very difficult to determine) and the use of "dry break" fittings (such fittings leave a dangerous quantity of liquid in the tank truck fill hose).